Resistance-bridge security system detecting resistance changes in either direction

ABSTRACT

An electronic security system capable of monitoring a plurality of stations for an intrusion. A current balancing unit consists of two balanced electronic detection systems which can recognize an increase or decrease of resistance at any of said stations, said increase and/or decrease in resistance created by an intrusion. Upon such a detection, the electronic system becomes unbalanced and a current will flow to operate periphery equipment which sounds an alarm or otherwise warns of the intrusion.

United States Patent Hall, Jr. [451 Mar. 28, 1972 54] RESISTANCE-BRIDGE SECURITY 3,399,393 8/1968 Becker et al ..340/285 SYSTEM DETECTING RESISTANCE 3,407,400 10/1968 CHANGES IN EITHER DIRECTION 3 223 223 2x32: [72] Inventor: Roy 0. Hall, Jr., 2273 7th Street, 3:530:373 9/1970 Coyahoga, Ohio 44221 g 221 Filed: Dec. 11, 1969 Z' 'f a sslstant xammeren wann, [21] Appl. N 884,120 Attorney-Mack D. Cook, ll

52 us. Cl. ..340/285, 323/75 F, 324/65 B, [571 ABSTRACT 340/276 An electronic security system capable of monitoring a plurali- [51] Int. Cl. ..G08b 13/00 ty of Stations for an intrusion A current balancing unit com of Search sists f two balanced electronic detection systems which can 323/75 75 75 324/65 330/146 recognize an increase or decrease of resistance at any of said 5 6 R f d stations, said increase and/or decrease in resistance created by E 1 e erences an intrusion. Upon such a detection, the electronic system UNITED STATES PATENTS becomes unbalanced and a current will flow to operate eri he e ui ment which sounds an alarm or otherwise 2,901,740 8/1959 Cutsogeorge ..323/75 E g g g g f 3,068,388 12/1962 Burski 3,156,778 11/1964 Cirone ..32 3/75 E 2 Claims, 3 Drawing Figures Q/II FIG 3 ROY o. HAL ,JR.

' W Maw ATTORNEYS RESISTANCE-BRIDGE SECURITY SYSTEM DETECTING RESISTANCE CHANGES IN EITHER DIRECTION BACKGROUND OF THE INVENTION This invention relates to an electronic security system for protection against theft. The system is designed in a foolproof manner so as to detect not only the unauthorized breaking and/or entering by a thief, but also the tampering with the system by a thief who knows a system exists and is attempting to deactivate the same.

Many electronic security systems have been designed for the protection of home and business. Those of the most simple nature merely set off an alarm when the circuit is broken. Within this classification are the type of alarm systems wherein a very fine or thin conductor is embedded in the glass of a window so that when the window is broken the alarm is activated. Such a system, however, has an inherent weakness in that that the professional thief, who knows of or suspects the existence of the system, can avoid it merely by seeking out the wires and jumping" the same across the window so that breaking the window will not open the circuit.

More acceptable circuits have been devised using balance arrangements such as a Wheatstone Bridge which would set off an alarm when the circuitry was caused to become unbalanced due to an intrusion or the like. Such devices, however, are deficient in sensitivity, draw inordinant amounts of current through the balancing leg, are incapable of servicing a number of stations (window, doors and the like) simultaneously, and, like the system previously described, are capable of being avoided by the sophisticated thief.

SUMMARY OF THE INVENTION It is, therefore, a primary object of the present invention to provide an electronic security system which is not only capable of detecting intrusions and attempted intrusions but also foolproof in that attempts at deactivation will be detected.

It is another object of the present invention to provide an electronic security system, as above, which is stable in an inactive environment yet highly sensitive to any change from the inactive environment.

It is still another object of the present invention to provide an electronic security system, as above, which will not draw large amounts of current and therefore be conveniently and economically powered even by emergency batteries should there be a power line failure.

It is a further object of the present invention to provide an electronic security system, as above, which is capable of serving a plurality of stations sumultaneously while maintaining its high degree of sensativity.

These and other objects which will become apparent from the following specification are accomplished by means hereinafter described and claimed.

In general, an electronic security system according to the invention will detect intrusion at one or more stations being monitored and includes power supply means, a resistive unit at each said station, and current balancing means which provide a current to actuate operating meanswhich in turn control the alarm means signaling the intrusion. The current balancing means have variable resistance means and at least two diodes which will not conduct when the variable resistance means is regulated to adjustably equal the total resistance of the resistive units. One of the diodes is wired across the resistive units and the other diode is wired across the variable resistance means so that increases in the resistance of the resistive units will actuate the first diode and decreases in the resistance of the resistive units will actuate the second diode.

BRIEF DESCRIPTION OF THE DRAWINGS balancing unit shown 2 DESCRIPTION OF THE PREFERRED EMBODIMENTS An electronic security system constructed according to the concept of the present invention is shown in block form in FIG. 1 as comprising a current balancing unit A which is able to control a number of penetration or intrusion stations 8. Typical intrusion stations would be, for example, doors, windows, vaults, and the like. As will hereinafter be explained, a single current balancing unit A is capable of monitoring a plurality of intrusion stations B.

As will become evident, should the current in thebalancing unit A become unbalanced due to any type of intrusion at one or more of the stations B, a current will be sent to at least one standard current amplifier C which activates one or more operating means or devices D, such as a switch, electromechanical relay, silicon controlled rectifier, or combination thereof. The operating device D thus activates periphery equipment E which can comprise audio or visual alarms, motion picture or television cameras, or any other means capable of signaling and/or recording the fact that an intrusion has been detected. a

One form of a current balancing unit A according to the concept of the present invention is shown in FIG. 2 as having a direct current voltage supply 10, impressed across a series circuit consisting of a variable resistor 11, a number of resistive units (at each station B) which are located across terminals 12, and a floating resistor 13 which maintains the unit above ground 14. The resistive units that comprise the penetration or intrusion station B are designed in such a way that an intrusion will either add resistance across or open terminals 12. The number of units between terminals 12 is not a fixed quantity and will depend on the number of stations being monitored. Further, the ohmic value of the resistors across terminals 12 is not necessarily a fixed value. It is only important that the value of the variable resistor 11 can be balanced with the resistance of the stations B so that the voltage at point 15 is equal to onehalf of the supply potential 10.

Nor is the value of the voltage supply 10 fixed, it only being important that its value be coordinated with the voltage and current ratings of the zener diodes l6 and 17 so that with the resistor 11 regulated as described above to create the balanced condition, there will be insufficient voltage across either of the standard zener diodes l6 and 17 wired between point 15 and ground 14 and resistor 11, respectively, and no current will flow through lines 18 and 19 respectively to the current amplifiers C. While two current amplifiers are shown in FIG. 1 activated through lines 18 and 19, it is evident that one skilled in the art could adapt the circuitry disclosed herein so that lines 18 and 19 would activate a single current amplifier C which would control a single operating device D to activate one set of periphery equipment E, two sets of all such equipment being shown herein merely as a matter of choice.

Any attempt to bypass or short out a resistive unit at any intrusion station B will change the resistance value of the series circuit and thus change the voltage at point 15. A change in voltage will also result from an open circuit or increased resistance in the series. If, for example, a portion of the series is shorted, the potential at point 15 is changed and diode 17 will have sufficient potential across it, with current limited by resistors 20 and 21 in series, so that diode 17 will conduct heavily and develop sufficient voltage across resistor 20 to cause the current amplifier C to operate. If, on the other hand, the line between terminals 12 becomes open or partially open, the diode 1 6 is caused to draw additional current thereby developing voltage across resistor 22. This additional current is then fed through line 18 to the current amplifier C as described above. It should be noted that the resistors 20 and 22 represent the input impedance to the current amplifiers C and therefore depending upon the selection of the particular amplifier C, resistive elements as such may not be necessary.

It has been found that due to the sensitivity characteristics of presently known zener diodes, their use in the circuit of FIG. 2 places a restriction on the impedance of the series circuits which could limit the number of intrusion stations B that can be successfully monitored. Therefore, a slightly different balancing unit A is required when a great number of intrusion stations are to be monitored. Such a unit is shown in F IG. 3.

Just as in the FIG 2 embodiment, a direct current voltage supply is impressed across a series circuit consisting of a variable resistor 31, a number of resistive units (at each station B) located across terminals 32, and a floating resistor 33 which maintains the unit above ground 34. Again, an intrusion at any station will either add or subtract from the resistance between terminals 32.

After resistor 31 is adjusted so that the voltage at point 35 is equal to one-half of the supply potential 30, the input transistors 36 and 37 are adjusted to draw a minimum current and in the interest of sensitivity, this minimum current is not zero. It is desirable that the current through the collectors of transistors 36 and 37 is just enough to maintain a voltage across resistors 38 and 39, respectively, equal to the voltage at point 35 less the internal minimum current base to emitter voltage of the transistors when measurements are taken from the positive power supply line, (as at point 40) and the negative power supply line, (as to point 41) respectively.

The adjustment of this current through transistor 36 and resistor 38 is accomplished by setting the current through output transistor 42 and resistor 38 to a near zero level by regulation of a variable resistor 43 placed for design considerations hereinafter discussed in series with resistors 44 and 45. Likewise, the current through transistor 37 and resistor 39 is accomplished by setting the current through output transistor 46 and resistor 39 to a near zero level by regulation of a variable resistor 47 in series with resistors 48 and 49.

In order to protect transistors 36 and 37 in the event of maladjustment of the balance resistor 31 or the variable resistors 43 and 47 respectively, load resistors 50 and 51 are provided to limit the current through transistors 36 and 37, respectively.

With resistors 31, 43 and 47 properly adjusted to achieve the balance described above, output transistors 42 and 46 will draw little or no current, at least current insufficient through lines 52 and 53, respectively, to activate the current amplifiers C and will therefore be essentially nonconductive. Again, two current amplifiers are shown; however one skilled in the art could readily adapt the circuitry described herein to operate with one amplifier C. in a similar manner, it should be evident that one skilled in the art could select circuit parameters so that the transistors 42 and 46 would supply enough current themselves to operate the periphery equipment doing away with the necessity of current amplifying means. However, in the interest of sensitivity and balance, the system shown herein, as preferred, includes current amplifiers.

An intrusion at one or more of the penetration stations B located across terminals 32 will change the voltage at point 35, such a change being detected by either transistor 36 or 37. If, for example, an intrusion is to be accomplished by increasing the resistance between terminals 32 or by opening the circuit therebetween, the normally conducting transistor 37 will no longer draw a sufficient amount of current to maintain the voltage level across resistor 39, and transistor 46 will be forced to maintain this voltage drop across resistor 39 equal to the level of the base voltage of transistor 46 less the characteristic base to emitter potential of transistor 46.

It should be evident that the amount of current through transistor 46 is a function of the size of resistor 39, the collector circuit resistance, and the power supply voltage. Further the base current of transistor 46 will also flow through resistor 39, as well as through resistors 47 and 48 and the base to emitter junction of transistor 46, the additional current through resistors 47 and 48 tending to increase the voltage level at the base of'transistor 46 resulting in a somewhat lower base to emitter voltage than might be desirable for maintaining the collector current through line 53 at a required level. It is therefore good design to have a sufficient amount of original current through resistors 47, 48 and 49 so that the small additional base current of transistor 46 will have a minimum effect upon the base voltage thereof, as measured from oint 40.

If the intrusion should be effected by lowering t e resistance between terminals 32, as by shorting out one or more resistive units, transistors 36 and 42 will operate similar to transistors 37 and 46 just described except that the base of transistor 42 is held at its voltage level by the series resistances 43, 44 and 45, that level being a function of the original adjustment of resistor 43, as described. The collector current of transistor 42 is then utilized to operate a current amplifier C through line 52 to activate the periphery equipment E through an operating device D.

It should now be evident that through proper selection of circuit parameters and careful balancing, the circuitry described above will carry out the objectives of the present invention and otherwise improve the electronic detection art.

lclaim:

1. An electronic security system for detecting an intrusion at one or more stations being monitored, comprising power supply means; a resistive unit at each of the stations; current balancing means having variable resistance means and at least two diodes which will not conduct when said variable resistance means is regulated to adjustably equal the total resistance of said resistive units, the first of said diodes being wired across said resistive units and the second of said diodes being wired across said variable resistance so that increases in the resistance of said resistive units will actuate said first diode and decreases in the resistance of said resistive units will actuate said second diode, to provide an output current; operating means actuated by the output current of said current balancing means; and, alarm means controlled by said operating means to selectively signal and record an intrusion.

2. An electronic security system according to claim 1 wherein said supply means is direct current. 

1. An electronic security system for detecting an intrusion at one or more stations being monitored, comprising power supply means; a resistive unit at each of the stations; current balancing means having variable resistance means and at least two diodes which will not conduct when said variable resistance means is regulated to adjustably equal the total resistance of said resistive units, the first of said diodes being wired across said resistive units and the second of said diodes being wired across said variable resistance so that increases in the resistance of said resistive units will actuate said first diode and decreases in the resistance of said resistive units will actuate said second diode, to provide an output current; operating means actuated by the output current of said current balancing means; and, alarm means controlled by said operating means to selectively signal and record an intrusion.
 2. An electronic security system according to claim 1 wherein said supply means is direct current. 